The present invention relates generally to an improved interface pad or layer for thermal management when used in combination with solid state electronic components or other types of heat generating electronic devices, and more particularly to an interface layer or pad which is adapted to be interposed along a heat dissipating path between a solid state electronic device and a mounting chassis or heat-sink surface. The interfaces of the present invention comprise a polyphenylsulfone (PPSU) layer, preferably but not necessarily loaded with a particulate solid, which is coated with conformal coatings (which are preferably loaded with thermally conductive fillers), on one or both sides in any combination. The polyphenylsulfone layer serves as a dielectric in those applications requiring heat transfer from a heat-generating semiconductor device or circuit to a normally highly conductive heat spreader, chassis, sink, or the like. When compounded or blended with thermally conductive fillers such as alumina, boron nitride, aluminum nitride, silicon carbide, silicon nitride, diamond, silver, copper, or the like, the thermal properties of the dielectric composite are enhanced over those already available from the polysulfone resin. In other words, the particulate solid or filler functions as an enhancer of thermal conductivity for the polyphenylsulfone resin matrix. The polyphenylsulfone in the formulations of the present invention has a glass transition temperature ranging from between about 200.degree. C. and 230.degree. C., and preferably a glass transition temperature of about 220.degree. C. The glass transition temperature of individual polymers is relatively narrow, thereby contributing to consistency and reliability of performance for layers or pads prepared from these formulations The high glass transition of these polymers allows them to retain good mechanical and electrical properties over a large temperature range, thereby rendering products made pursuant to the present invention ideal candidates for applications requiring heat conducting electrically insulative interfaces.
The polyphenylsulfone of the interface layers or pads is advantageous due to the excellent thermal properties and high temperature performance characteristics with low water absorption properties as well. The high temperature properties permit utilization of the layers under continuous high temperature conditions. Additionally, the polyphenylsulfone resin is highly ductile and hence less prone to cracking or embrittlement. Polyphenylsulfones possess high chemical and solvent resistance and facilitate rapid and convenient production operations in the molten or melt stage. The interfaces prepared in accordance with the present invention are durable and exhibit a high cohesive strength. At the same time, the material possesses good cut-through properties, thereby enabling a wide variety of production techniques to be used in shaping and/or configuring into products.
The interfaces of the present invention may be secured or attached either directly onto the surface of the solid state electronic device or directly mounted to the surface upon which the device is being operatively coupled. These interfaces may be secured as required onto surfaces of substrates of metallic, ceramic, or polymeric materials. Typically, the polyphenylsulfone materials of the present invention will be coated on one or both sides with conformable elastomeric films or coatings based on silicone, polyurethane or other elastomeric polymers. Other coatings may include pressure sensitive adhesives such as silicone, acrylic, thermoplastic elastomer types, and the like. Heat activated "dry" adhesives such as polyimide(amide), polyurethane, epoxy adhesives and the like may be utilized as well. Phase-change and/or hot-melt coatings which are typically dry at room temperature but flow at device utilization temperatures may also be satisfactorily employed. Additionally, the conformable elastomeric coatings may be filled with thermally conductive fillers such as alumina, boron nitride, aluminum nitride, silicon carbide, silicon nitride, diamond, silver, copper, and the like. These coated layers or films of polyphenylsulfone are then interposed between a heat generating solid state device or circuit, and a heat sink/spreader/chassis. These layers also find application as a dielectric member in etched copper circuit configurations where they would be interposed between the copper circuitry and a heat spreading substrate, typically fabricated from aluminum.
The significant advantage of utilizing polyphenylsulfone polymer is that it is naturally thermally conductive. Films (1, 5 and 10 mil) made from polyphenylsulfone, available from Amoco (Chicago, Ill.) as R-5100 grade, were tested in accordance with ASTM-D5470 method. These films exhibited a thermal conductivity of 0.25 W-m.sup.-1 K.sup.-1 which is twice the thermal conductivity of commonly employed engineering thermoplastics. Examples of the commonly employed engineering plastics are polyimide (PI from E.I. DuPont deNemours Corp. of Wilmington, Del.), polyester (PET, PEN from DuPont), polyphenylenesulfide (PPS from Toray Engineering, Ltd. of Tokyo, Japan or Philips Petroleum of Bartlesville, Okla.), and polyetherimide (PEI from GE Plastics of Pittsfield, Mass.). In the table below we list some salient properties of these engineering plastics, to illustrate that PPSU has excellent engineering properties in addition to its high thermal conductivity.
TABLE I __________________________________________________________________________ PI PPS PEI PEN Property Kapton Torelina Ultem Kaladex PPSU __________________________________________________________________________ Thermal 0.11 0.13 0.12 0.15 0.25 Conductivity (W/mK) Tensile Strength 33500 39000 14200 32000 10100 (psi) Tensile Modulus 370000 570000 475000 870000 340000 (psi) Glass Transition &gt;350 90 215 120 220 (.degree. C.) Water Absorption 2.2 0.05 0.25 0.4 0.37 (%) Electrical 10.sup.18 5 .times. 10.sup.17 10.sup.17 10.sup.18 &gt;10.sup.15 Resistivity (ohm-cm) Electrical 220 180 170 180 180 Continuous Use Temperature (.degree. C.) __________________________________________________________________________
With the addition of carefully chosen amounts of thermally conductive fillers like alumina, aluminum nitride, boron nitride, silicon carbide, etc., it is possible to increase the thermal conductivity of the polyphenylsulfone matrix even further while maintaining satisfactory mechanical and electrical properties.
The features of the present invention provide a highly thermally conductive interface which is attached to surfaces along a thermal path, with the improved interface being a highly thermally conductive dielectric having a consistent and uniform thickness free of air and/or voids. This combination of features contributes to and results in consistency of performance. Given this capability in the thermally conductive interface or pad, greater predictability of performance is available from semiconductor devices utilized in combination with the pad. These advantages are obtained without experiencing the problems inherent in applications of silicone grease.
A common technique used in the past has been to prepare and fabricate a thermally conductive, electrically insulative pad of a polyimide(amide) such as Kapton or the like. Polyimide(amide) films are commercially available under the trade designation "KAPTON" from E.I. DuPont deNemours Corp. of Wilmington, Del. Particulate-filled Kapton materials have been utilized in the past, and have proven to perform satisfactorily. While these filled polyimide(amide) layers or pads perform reasonably well, they have a relatively high moisture absorption which tends to reduce the dielectric properties. In addition the Polyimide(amide) films tend to be expensive. The pads prepared in accordance with the present invention and employing polyphenylsulfone, particularly particulate-filled polyphenylsulfone are unique and exhibit unexpectedly good thermal performance characteristics. As indicated, the combination of mechanical, electrical, and thermal properties permits continuous utilization of the interfaces or pads at high operating temperatures.